1) Field of the Invention
This invention relates to the synthesis of metal oxides of high specific surface area and high porosity which are stable to heat.
2) Background Art
For use as an adsorbent and especially as a catalyst support, it is important to have available metal oxides of high specific surface area and high porosity which retain their specific surface area and their porosity during prolonged heating at elevated temperature. In fact, the lifetime of the catalysts in part depends on their resistance to heating, and the lifetime of the catalysts is a factor determining the economy of catalytic processes. The use of a stable catalyst support makes it possible to reduce to a minimum the plant shutdowns for replacement of spent catalyst.
The synthesis of these oxides by calcination of metal salts requires high temperatures. For some metals, such as zirconium, the calcination of its salts at high temperature results in the formation of oxides having a specific surface area which does not exceed 2-3 m.sup.2 /g.
Other oxides may be obtained by calcination, in general of the corresponding hydroxides. Thus, aluminas are obtained by heating various aluminium hydroxides.
The temperature stability of these oxides and therefore the lifetime of the catalysts which contains them is still far from being satisfactory (sic).
Numerous processes have been proposed for the production of highly porous catalyst supports, the porosity of which withstands prolonged heat treatments.
Thus, according to U.S. Pat. No. 4,731,352, metal oxides such as alumina, silica, zirconium oxide or titanium oxide are combined with an olefin/maleic anhydride copolymer. This combination produces a significant and stable porosity. However, the process is lengthy and costly to carry out and proceeds via the dissolution of a metal salt and the formation of a gel.
According to French Patent Application 2,540,887, a stable zirconium oxide is obtained by coprecipitation of a solution containing a zirconium compound and a stabilizing metal compound, followed by drying and calcination.
Russian Patent SU-A-395328 describes the preparation of porous zirconium by oxidation with alkali metal carbonates in a mixture of molten alkali metal chlorides. As the melting point of this mixture is very high, the crystallites obtained have a large size, of around 3000 nm, and thus a low specific surface area. It should also be mentioned that this patent gives no indication whatsoever with regard to the specific surface areas or the porosities of the solids obtained.
We have now found a process for simple synthesis, in a single step, of metal oxides of high specific surface area and high porosity. The temperatures used in this process are very much lower than the customary calcination temperatures, which makes the process more economical. The oxides of high specific surface area and high porosity which are obtained are stable at high temperature and permit excellent dispersion of the catalytically active phases.